Surface elastic wave filter with resonant cavities

ABSTRACT

A surface elastic wave filter has resonant cavities and comprises a composite substrate formed of a base substrate and a piezoelectric upper layer; at least one input electroacoustic transducer and an output electroacoustic transducer, arranged on the upper layer, and at least one internal reflecting structure, arranged between the input electroacoustic transducer and the output electroacoustic transducer. The internal reflecting structure comprises a first structure comprising at least one reflection grating having a first period and a second structure comprising at least one reflection grating having a second period, the first period being greater than the second period.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Patent Application PCT/FR2022/051736, filed Sep. 15, 2022,designating the United States of America, which claims the benefit underArticle 8 of the Patent Cooperation Treaty to French Patent ApplicationSerial No. FR2110135, filed Sep. 27, 2021.

TECHNICAL FIELD

The present disclosure relates to a surface acoustic wave filter withresonant cavities. Such a filter finds its application in the field ofthe processing of high-frequency signals, for example, intelecommunications applications.

BACKGROUND

With reference to FIG. 1 , and as is known, for example, from EP0608249,such a filter 1 typically comprises, arranged on a piezoelectricsubstrate, at least two, respectively input and output, electroacoustictransducers 2 a, 2 b. The two transducers define a path along whichsurface elastic waves propagate. They also form the electrical inputports Pe and output ports Ps of the filter 1.

The filter 1 can comprise two external reflection gratings 3 a, 3 b (andpreferably only reflecting), typically Bragg mirrors, arranged on eitherside of the pair of input 2 a and output 2 b transducers, along the axisX of propagation of the elastic waves.

The filter 1 comprises at least one internal reflection grating 4,arranged between the two transducers 2 a, 2 b, on the path X of theelastic waves and which can be equidistant from these two transducers 2a, 2 b. This at least one internal reflection grating 4 has a non-zerotransmission coefficient.

A plurality of resonant cavities C1, C2 are defined in this way alongthe wave propagation path, these resonant cavities forming an equalnumber of poles of the filter 1.

Document DE2363701 also discloses several configurations of a surfaceelastic wave filter with resonant cavities.

Document WO2020021029 proposes numerous advantageous configurations ofsuch a surface elastic wave filter with resonant cavities, theseconfigurations all being made on a composite substrate, that is to say asubstrate formed from a base substrate (constituting a mechanicalsupport) and a piezoelectric upper layer. The elastic waves propagate inthe upper layer in the form of longitudinal and/or shearing waves. Thethickness of the upper layer is chosen to be on the order of magnitudeof the central wavelength of the filter, or less than this wavelength.The propagation then occurs without losses of radiation in the basesubstrate.

The very general structure of the filter proposed by the aforementioneddocument makes it possible to synthesize a wide variety of band-passfilters having a low loss of insertion (<2 dB) in the passband and asignificant rejection rate (>15 dB) outside this band.

It is well known (see, for example, EP0608249) that the synthesis of asurface elastic wave filter with resonant cavities can cause theappearance of secondary lobes in the filter rejection or transitionband. These lobes are of amplitudes even higher when it is sought towiden the relative passband of the filter.

It would therefore be desirable to have a filter structure that canlimit the amplitude of the lobes present in the rejection or transitionband.

BRIEF SUMMARY

One aim of the present disclosure is to propose a filter structuremaking it possible to address this desire. More specifically, an objectof the present disclosure is to propose a filter structure that reducesthe amplitude a height of the lobes present in the rejection ortransition band relative to the filter structures of the state of theart.

With a view to achieving this aim, the object of the present disclosureis to propose a surface elastic wave filter with resonant cavitiescomprising:

-   -   a composite substrate formed of a base substrate and a        piezoelectric upper layer;    -   at least one input electroacoustic transducer and an output        electroacoustic transducer, arranged on the upper layer;    -   at least one internal reflective structure, arranged between the        input electroacoustic transducer and the output electroacoustic        transducer.

The internal reflective structure of the filter comprises:

-   -   a first structure comprising at least one reflection grating        having a first period;    -   a second structure comprising at least one reflection grating        having a second period, the first period being greater than the        second period;

According to the present disclosure, the first structure comprises afirst plurality of reflection gratings separated from one another by afirst distance, the second structure comprises a second plurality ofreflection gratings separated from one another by a second distance, andthe first distance is less than the second distance.

According to other advantageous non-limiting features of the presentdisclosure, taken alone or according to any technically feasiblecombination:

-   -   the first structure is distant from the input electroacoustic        transducer by a first separation distance, the second structure        is distant from the output transducer by a second separation        distance, the first separation distance being less than the        second separation distance;    -   the filter further comprises two external mirrors arranged on        either side of the input and output transducers;    -   the reflection gratings and/or the external mirrors are made by        arrays of metal fingers disposed on/in the top layer of the        composite substrate;    -   the reflection gratings and/or the external mirrors are made by        arrays of grooves etched into the composite substrate;    -   the composite substrate comprises at least one layer arranged        between the base substrate and the piezoelectric upper layer;    -   the base substrate has an electrical resistivity of greater than        1000 ohm·cm;    -   the thickness of the piezoelectric upper layer is less than 20        microns;    -   the input electroacoustic transducer and the output        electroacoustic transducer are respectively comprised of two        interdigitated comb electrodes;    -   the distance separating the first structure and the second        structure is equal to the arithmetic mean, within 10%, of the        first distance and the second distance.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present disclosure will emerge fromthe following detailed description of the present disclosure withreference to the accompanying figures, in which:

FIG. 1 shows a surface elastic wave filter with resonant cavities of theprior art;

FIGS. 2A and 2B show, in schematic top and sectional view, a surfaceelastic wave filter with resonant cavities according to the presentdisclosure;

FIG. 3A shows a filter whose structure is compliant with the prior art;

FIG. 3B shows the module of the transfer function of a filter accordingto a first counterexample;

FIG. 3C shows the structure of a filter according to the presentdisclosure;

FIG. 3D shows the module of the transfer function of a filter of a firstexample embodiment of the present disclosure;

FIG. 4A shows the module of the transfer function of a filter accordingto a second counter-example;

FIG. 4B shows the module of the transfer function of a filter of asecond example embodiment of the present disclosure.

DETAILED DESCRIPTION

For the sake of simplifying the following description, the samereferences are used for elements that are identical or perform the samefunction in different disclosed embodiments of the present disclosureand in the prior art.

The filter that will be described in the rest of this description iscarried out on a composite substrate formed of a base substrate and ofan upper piezoelectric layer, which rests on this base substrate. Otherlayers may be provided between these two elements. It may, for example,be one or a plurality of amorphous layer(s) facilitating their assembly.This amorphous layer (or the plurality of amorphous layers) may be, orcomprise, silicon oxide. The thickness of the amorphous layer may begreater than or less than 200 nm in thickness, in particular, it may bechosen between 10 nm and 6 microns. A Bragg mirror formed of a stack oflayers with an elastic impedance periodically alternating is possiblebetween the upper layer and the silicon substrate.

The piezoelectric upper layer of the composite substrate may consist ofaluminum nitride (AlN), zinc oxide (ZnO), PZT, potassium niobate (KNbO₃)and similar materials such as KTN, PMN-PT and related materials, galliumnitride (GaN), lithium tantalate (LiTaO₃) or lithium niobate (LiNbO₃).Preferably, the crystallographic orientation of lithium tantalate(LiTaO₃) or lithium niobate (LiNbO₃) is defined as (YXl)/θ according tothe IEEE 1949 Std-176 standard, θ being a crystallographic orientationangle comprised between 0° and 60° or between 90° and 150°, Y and Xbeing the crystalline axes, and l representing the length of a platecollinear to X around which the rotation θ is carried out.

The base substrate of the composite substrate may be a high electricresistivity substrate, in particular, a silicon substrate. Highresistivity means an electrical resistivity of more than 1000 ohm·cm.Provision may be made for the base substrate to comprise an upper layerfor trapping electrical charges, for example, a layer of polycrystallinesilicon. The composite substrate may also be made of diamond carbon,sapphire, glass or silicon carbide.

The composite substrate may be formed in multiple ways, and, inparticular, via Smart Cut™ technology. According to this technology, athin layer to be transferred is defined in a piezoelectric donorsubstrate by ion implantation of lightweight species (hydrogen and/orhelium, for example) through a main face of this donor substrate. Thisimplantation forms a fragile zone defining the thin layer with the mainface of the substrate. The donor substrate is assembled, by its mainface, to the base substrate, and the donor substrate is fractured at thefragile zone in order to transfer the thin layer, for example, using aheat and/or mechanical treatment.

Alternatively to this approach, and, in particular, when thepiezoelectric upper layer of the composite substrate is relativelythick, on the order of 5 microns to 20 microns or more, the compositesubstrate can be formed by assembling a donor substrate to the basesubstrate and thinning the donor substrate, for example, by CMP,grinding, or polishing.

In all cases, and regardless of the approach chosen, the transfer of thethin layer leads to forming a composite substrate formed of amonocrystalline piezoelectric upper layer on the base substrate.

The thickness of the piezoelectric upper layer is on the order ofmagnitude of the length of the elastic wave, which is intended topropagate therein. Preferably, the thickness of the piezoelectric upperlayer is less than this wavelength, for example, about 20 microns orless. The thickness of the base substrate is itself much greater thanthe thickness of the piezoelectric upper layer, and typically at leastten times or one hundred times greater than this layer, for example,between 250 microns and 500 microns.

FIGS. 2A and 2B show a surface elastic wave filter 1 with resonantcavity, which is the subject of the present disclosure. This filter 1 isproduced on or in the upper piezoelectric layer S1 of the compositesubstrate S, in a conventional manner, here assembled to a support S3via an assembly layer S2. Reference may be made to the work “Surfaceacoustic wave filters with application to electronic communications andsignal processing” from David P. Morgan Academic Press; 2^(nd) edition(Aug. 9, 2007) to obtain the general principles allowing the synthesisof such a surface elastic wave filter.

The filter is a bandpass filter, which has a central frequency f₀ andpassband Δf, which can be expressed as a fraction of this centralfrequency, typically between 0.1% and 10%.

The filter 1 comprises at least one input electroacoustic transducer 2 aand an output electroacoustic transducer 2 b. The electromagnetic signalto be filtered is applied to the input electroacoustic transducer 2 aand the filtered electromagnetic signal is taken from the outputelectroacoustic transducer 2 b. These two transducers thereforerespectively form an input port and an output port of the filter.However, it should be noted that the “input” and “output” designation istotally arbitrary and that the filter can be exploited byapplying/presampling an electromagnetic signal on either of the twoports.

Very generally, the transducers 2 a, 2 b are in accordance with thoseknown in the prior art, a few features of which are listed below.

Here each transducer 2 a, 2 b consists of two interdigitated combelectrodes. As is well known, per se, such transducers 2 a, 2 b consistof an array of metal fingers, which are alternately connected to twobuses between which a difference in electrical potential isapplied/removed. With this device it is possible to directlygenerate/detect a surface elastic wave in the piezoelectric upper layer.The input 2 a and output 2 b transducers are generally configured to beidentical, but it is not out of the question for them to differ, inparticular, in order to optimize the response of the filter.

The metal of the electrodes is typically based on aluminum, for example,pure aluminum or an aluminum alloy such as aluminum doped with Cu, Si orTi. It is nevertheless possible to use another material, for example, toestablish a higher reflection coefficient for a lower relative electrodethickness. In this regard, the preferred electrode materials are copper(Cu), molybdenum (Mo), nickel (Ni), platinum (Pt) or gold (Au) with anadhesion layer such as titanium (Ti) or tantalum (Ta) or chromium (Cr),zirconium (Zr), palladium (Pd), iridium (Ir), tungsten (W), etc.

The period p of the electrodes is generally chosen such that p=λ/2, λbeing the wavelength of the elastic wave, which propagates in thepiezoelectric upper layer at the resonance frequency f_(r) of thetransducer. Other configurations are possible, and more generally theperiod p can be written as p=λ/(nb_elec), with nb_elec being the numberof fingers (electrodes) per wavelength. These parameters are linked bythe relationship λ=V/f_(r), wherein V represents the phase velocity ofthe elastic wave under the transducer. The speed of the elastic wave onthe free surface or under an electroacoustic transducer depends, inparticular, on the nature of the material in which the wave propagates,and it is generally known or accessible to the person skilled in theart.

In the case of the band-pass filter, which is the subject of thisdescription, the resonance frequency f_(r) is set in the passband of thefilter or, preferably, at a lower frequency than the lower bound of thisband. By setting this resonant frequency, the wavelength λ of the wavecan be determined, and therefore so can the period p of the transducer.

The metallization width of the electrodes, denoted a, is generallychosen so that the a/p ratio is of the order of 0.5. The number of pairsof fingers of each transducer is usually chosen in the order of 20 or100. Generally, increasing the number of pairs of fingers makes itpossible to increase the rejection of the frequencies not in thepassband.

Returning to the general description of the filter shown in FIGS. 2A-2B,it is possible to provide two external mirrors 3 a, 3 b arranged oneither side of the pair of the input 2 a and output 2 b transducers. Asis well known, these mirrors 3 a, 3 b make it possible to confine asmuch of the elastic energy as possible between the two input and outputelectroacoustic transducers 2 a and 2 b. As such, they are designed tohave a very high reflection coefficient, as close as possible to 1(total reflection of the incident energy), by choosing the metalthickness of the fingers of the mirrors and the number of these fingers,typically a few tens per mirror in the case of a composite substrate fora shear wave. It should however be noted that these two external mirrorsare in no way essential to the present disclosure, and that a filteraccording to the present disclosure and entirely functional can bedevoid of them.

To finish the description of the filter shown in FIG. 2A-2B, the filteralso comprises, arranged between the input electroacoustic transducer 2a and the output electroacoustic transducer 2 b, at least one internalreflecting structure. The reflecting structure consists of a pluralityof reflection gratings R1, R2 distributed along the path of propagationof the elastic waves. As has been recalled in the introduction of thepresent disclosure, this structure defines a plurality of resonantcavities along the propagation path of the elastic waves, these resonantcavities forming an equal number of poles of the filter 1.

Each reflection grating R1, R2 of the internal reflecting structure isformed of Bragg reflectors composed of metal fingers, preferentiallyshort-circuited. The period of the array of fingers that constitute eachreflection grating R1, R2 is generally chosen to be close (within 15%)to the period of the electrodes of the transducers, and thereforegenerally close to λ/2. By choosing the period of the reflectiongratings R1, R2 that are less than that of the transducers, the signalswhose frequency is greater than that of the passband of the filter willbe better rejected, that is to say the presence of secondary lobes willbe pushed back into higher frequencies. Conversely, by choosing theperiod of the reflection gratings R1, R2 that is greater than that ofthe transducers, the signals whose frequency is lower than that of thepassband of the filter will be better rejected, that is to say thepresence of secondary lobes will be pushed back into lower frequencies.

The number of metal fingers of each reflection grating R1, R2 determinesthe reflection coefficient of the grating, whose maximum value is equalto 1 (total reflection of the incident wave). A high-reflectioncoefficient, greater than 0.5 or greater than 0.8, makes it possible toincrease the frequency rejection outside the passband. However, itreduces the passband, which may be compensated, if this reduction isexcessive, by adding a reflection grating to the internal reflectingstructure (and therefore a pole in the filter). Typically, this numberof fingers of each reflecting structure may be between 10 and 40.

In the filter 1, which is the subject of the present disclosure, theinternal reflecting structure comprises:

-   -   a first structure Pright comprising at least one reflection        grating R1 having a first period;    -   a second structure Pleft comprising at least one reflection        grating R2 having a second period, the second period being        distinct from the first period.

By convention, it will be necessary that the first period, thereforecorresponding to the period of the reflection grating R1 of the firststructure Pright, is greater than the second period, thereforecorresponding to the period of the reflection grating R2 of the secondstructure Pleft. “Greater” means that the first period is greater thanthe second period by at least 5 nm.

Advantageously, in order to provide more flexibility in the design ofthe filter, the first structure Pright has a plurality of reflectiongratings R1, and each reflection grating may have the same first period.Similarly, the second structure Pleft also comprises a plurality ofreflection gratings R2 and each reflection grating may have the samesecond period. In all cases, the periods of the reflection gratings ofthe first structure Pright are greater than the periods of thereflection gratings of the second structure Pleft.

When the first structure Pright has a plurality of reflection gratingsR1, these gratings are separated from one another by a first distanceRg. Similarly, when the second structure Pleft has a plurality ofreflection gratings R2, these gratings are separated from one another bya second distance Lg. In this configuration, the first distance Rg isless than the second distance Lg.

Preferentially, the first distance Rg is chosen to be less than athreshold distance Dg, called “Bragg distance” in the presentdescription. At the same time, the second distance Lg is chosen to begreater than the Bragg distance Dg. The first distance Rg and the seconddistance Lg separating the reflection gratings R1, R2 in, respectively,the first structure Pright and the second structure Pleft, define thedimensions of the acoustic cavities.

In the case where the period of the reflection gratings is chosen closeto λ/2, the Bragg distance is substantially equal to λ/4. Moreprecisely, the Bragg distance Dg is defined by the relationshipDg=(V′*p₀)/(2*V) wherein V represents the phase velocity of the elasticwave under the reflection gratings, V′ the phase velocity of the freesurface elastic wave and p₀ the period of a reflection grating whosemaximum reflection would be equal to the central frequency f0 of thefilter.

The first distance Rg can be chosen less than the Bragg distance Dg by afactor Fr of between 0.5 and 1, that is to say Rg=Fr*Dg. Similarly, thesecond distance Lg can be chosen greater than the Bragg distance Dg by afactor Fl of between 1 and 2, that is to say Lg=Dg*Fl.

The distance Pcg separating the first and the second structure may bechosen equal to the arithmetic mean of the first and the second distanceRg, Lg. “Equal” means equal to within 10%.

Also preferentially, the first structure Pright is distant from theinput electroacoustic transducer 2 a by a first separation distance Srg,the second structure Pleft is distant from the output transducer 2 b bya second separation distance Slg, and the first separation distance Srgis less than the second separation distance Slg.

The number of reflection gratings R1, R2 in the first and the secondstructure Pleft, Pright may be chosen with great freedom. It defines thenumber of elastic cavities (in addition to those defined above by thefirst and the second separation distance Srg, Slg) and therefore thenumber of poles of the filter. A relatively large number of reflectiongratings R1, R2 therefore makes it possible to conform the behavior ofthe filter to a predetermined template with great flexibility. It is notnecessary to have the same number of reflection gratings R1, R2 in thetwo structures Pleft, Pright. It is typically possible to choose,according to the filtering constraints provided by the determinedtemplate, between 2 and 10 reflection gratings R1, R2 in each of thestructures Pleft, Pright. The factors limiting the number of reflectivestructures R1, R2 will be ohmic losses and acoustic propagation.

By proposing a filter structure that has the features described above,it has been observed that it was then possible to propose a band-passfilter whose secondary lobes in the rejection or transition band wereparticularly reduced (for example, a reduction of at least 5 dB andtypically of the order of 10 dB) even for a relatively high passband, ofmore than 1% relative width.

This very advantageous behavior of such a filter is made apparent bycomparing, in the sections that follow in the description, the transferfunction of a conventional filter (counter-example) to the transferfunction of a filter synthesized using a structure according to thepresent disclosure (example).

In all the examples and counterexamples that follow, the compositesubstrate S comprises a layer S2 of silicon oxide that is 500 nm thickarranged between a piezoelectric layer S1 of LiTaO3 (YXl)/42° that is0.6 microns thick and a base substrate S3 of silicon (100).

In such a composite substrate S, an elastic wave having the wavelengthstargeted in these examples mainly propagates in the form of shear waves.

Counter-Example 1: Filter Having a Central Frequency of 400 MHz and 0.4%Relative Passband

At the central frequency of the filter, and in the previously definedcomposite substrate, the wavelength of the elastic wave propagating inthe upper layer S1 is 10.9 microns.

FIG. 3A shows a filter 1′ whose structure is compliant with the priorart. It has, formed on the composite substrate S, an inputelectroacoustic transducer 2 a and an output electroacoustic transducer2 b and an internal reflecting structure composed here of fourreflection gratings R separated from each other by an equal distance ofthe Bragg distance Dg. The filter 1′ further comprises two externalmirrors 3 a, 3 b arranged on either side of the input and outputtransducers 2 a, 2 b.

The parameters of the filter 1′ taken in counter-example 1 were obtainedat the end of a series of simulations of a model of this filter aimed atshaping its transfer function to a predetermined band-pass template G,shown in FIG. 3B.

Thus, the period p of the input and output electroacoustic transducers 2a, 2 b has been chosen at 5.450 microns. The metallization width of theelectrodes is chosen so that the a/p ratio is set to 0.6. The number ofpairs of metal fingers of the input electroacoustic transducer 2 a andan output electroacoustic transducer 2 b is set at 35.

The period of each external mirror is set at 5.557 microns. Eachexternal mirror has 40 fingers and an a/p ratio of 0.5. The distanceseparating, respectively, the external mirrors of the inputelectroacoustic transducer and the output electroacoustic transducer isset to 2.110 microns.

The reflection gratings R are separated from one another by a distanceDg of 2.938 microns, close to a Bragg distance of a quarter of awavelength. The first and the last of these reflection gratings arespaced apart, respectively, from the input electroacoustic transducerand from the output electroacoustic transducer by a distance of 3.36microns. The reflection gratings R each have 30 metal fingers.

The periods of these reflection gratings 4 are all identical and equalto 5.557 microns.

FIG. 3B shows the module of the transfer function of this filter 1′, aswas obtained at the end of the simulations having led to its synthesis.It is observed that this transfer function conforms well to the templatein the passband, but the presence of a plurality of lobes ofparticularly high amplitudes in the rejection band is also observed. Thepresence of these lobes is not desirable, as has been reported by theprior art document cited in the introduction of the present disclosure.

Example 1: Filter Having a Central Frequency of 400 MHz and 0.4%Relative Passband

FIG. 3D shows the module of the transfer function of a filter 1 havingthe structure shown in FIG. 3C conforming to the structure of a filterof the present disclosure.

The parameters of the filter 1 of example 1 were obtained by simulation,just like those of counter-example 1. Like the filter 1′ ofcounter-example 1, the filter 1 also has four reflection gratings R1,R2, two being arranged in the first structure Pright and the other twobeing arranged in the second structure Pleft. The reflection gratingsR1, R2 each have 30 metal fingers. The other parameters obtained bysimulation of this filter 1 are as follows:

First Structure Pright:

-   -   Srg: 2.645 microns    -   Rg: 1.446 micron, which is much lower than the Bragg distance Dg        of 2.938 microns    -   Period of the 2 reflection gratings R1 (the first period): 5.686        microns

Second Structure Pleft:

-   -   Slg: 4.350 micron    -   Lg: 4.790 microns, which is much higher than the Bragg distance        Dg of 2.938 microns.    -   Period of 2 reflection gratings R2 (the second period): 5.399        microns.

It can be seen that the first period of the reflection gratings R1 ofthe first structure Pright is much greater than the second period of thereflection gratings R2 of the second structure Pleft.

The distance Pcg separating the first structure Pright from the secondstructure Pleft is 3.125 microns, substantially equal to the arithmeticmean of the first distance Rg and the second distance Lg (3.118microns).

The input and output electroacoustic transducers 2 a, 2 b have the sameparameters as those of counter-example 1. The period of each externalmirror is fixed at 5.557 microns. Each external mirror has 40 fingersand an a/p ratio of 0.5. The distance separating, respectively, theexternal mirrors of the input electroacoustic transducer and the outputelectroacoustic transducer is set to 1.800 microns.

FIG. 3D shows the module of the transfer function of this filter 1, aswas obtained by simulation. It can be seen that the transfer functionconforms well to the template in the passband. In addition, and althoughlobes are still present in the rejection band, they are of smaller andrejected amplitudes at frequencies farther from the passband.

The performance of the filter 1 of this example 1 is therefore muchgreater than the performance of the filter 1′ of counter-example 1, thestructure of which conforms to a filter structure of the prior art.

Counter-Example 2: A Filter Having a Center Frequency of 1900 MHz and3.5% Relative Passband

It is now sought to synthesize a filter whose relative passband isclearly wider than that of example 1 and counter-example 1.

At the central frequency of the filter, and in the composite substrate Spreviously defined, the wavelength of the elastic wave propagating inthe upper layer S1 is 2.058 microns.

The structure of the filter of counterexample 2, identical to that ofcounter-example 1, is shown in FIG. 3A. The parameters of the filter 1′of the counter-example 2 were obtained at the end of a series ofsimulations of a model of that filter 1′ aimed at shaping its transferfunction to a predetermined template G, shown in FIG. 4A.

The period p of the input and output electro-acoustic transducers hasbeen chosen to be 1.029 microns. The metallization width of theelectrodes is chosen so that the a/p ratio is set to 0.54. The number ofpairs of metal fingers of each transducer is set to 16.

The period of each external mirror is set at 1.060 microns. Eachexternal mirror has 25 fingers and an a/p ratio of 0.5. The distanceseparating, respectively, the external mirrors of the inputelectroacoustic transducer and the output electroacoustic transducer isset at 0.301 micron.

The four reflection gratings R each have 10 metal fingers. The internalreflection gratings are separated by a distance Dg of 0.535 microns,corresponding to about a Bragg distance of a quarter-wavelength. Thefirst and the last of these reflection gratings are spaced apart,respectively, from the input electroacoustic transducer and from theoutput electroacoustic transducer by a distance Srg, Slg of 0.594microns.

The periods of these inner and outer reflection gratings 4 are allidentical and equal to 1.060 microns.

FIG. 4A shows the module of the transfer function of this filter, as wasobtained by simulation. It is observed that it conforms to thepredetermined template in the passband, but, as in counter-example 1,the presence of a plurality of lobes, of particularly high amplitudes inthe rejection band is observed.

Example 2: A Filter Having a Center Frequency of 1900 MHz and 3.5%Relative Passband

FIG. 4B shows the module of the transfer function of a filtersynthesized by simulation from a filter structure shown in FIG. 3C andin accordance with the structure of a filter of the present disclosure.

The four reflection gratings R1, R2 each have 11 metal fingers. Theother parameters also obtained by simulation of this filter are asfollows:

First Structure Pright:

-   -   Srg: 0.496 microns    -   Rg: 0.290 microns, which is much lower than the Bragg distance        Dg of 0.593 microns    -   Period of the reflection gratings R1 (the first period): 1.100        microns

Second Structure Pleft:

-   -   Slg: 0.767 microns    -   Lg: 0.827 microns, which is much higher than the Bragg distance        Dg of 0.593 microns.    -   Period of the reflection gratings R2 (the second period): 1.009        microns

It can be seen that the first period of the reflection grating R1 of thefirst structure Pright is much greater than the second period of thereflection grating of the second structure Pleft.

The distance Pcg separating the first structure Pright from the secondstructure Pleft is 0.56 microns, substantially equal to the arithmeticmean of the first distance Rg and the second distance Lg (0.558microns).

The period p of the input and output electro-acoustic transducers hasbeen chosen to be 1.0214 microns. The metallization width of theelectrodes is chosen so that the a/p ratio is fixed at 0.545. The inputtransducer has 16 pairs of fingers and the output transducer has 16pairs of fingers.

The period of each external mirror is fixed at 1.06 microns. Eachexternal mirror has 30 fingers and a ratio a/p of 0.5. The distanceseparating, respectively, the external mirrors of the inputelectroacoustic transducer and the output electroacoustic transducer isset at microns.

FIG. 4B shows the module of the transfer function of this filter, as wasobtained by simulation. It is clearly observed that it conforms to thetemplate in the passband. In addition, and although lobes are stillpresent in the rejection band, these are of much lower amplitudes, whichare rejected at frequencies farther from the passband.

The performance of this filter is therefore much higher than theperformance of the filter of counter-example 2, the structure of whichconforms to a filter structure of the prior art.

The synthesis of a filter according to the present disclosure can besimplified by breaking it down into several steps.

During a first step, a first incomplete filter is synthesized comprisingthe input electroacoustic transducer 2 a and the output electroacoustictransducer 2 b, and only one of the first structure Pright and thesecond structure Pleft. It is naturally possible to integrate theexternal mirrors in the simulations if such mirrors are provided in thecomplete filter 1. The simulations carried out during this first stepmake it possible to establish the parameters of the chosen structure,that is to say at least the distance separating the reflection gratingsR1, R2 included in this structure, and the number and configuration ofthese gratings.

During a second step, a second incomplete filter is synthesized, thistime incorporating only one, either the first or the second structurePright, Pleft, whichever was not selected during the first step. Thissecond step makes it possible to establish the parameters of this filterstructure, by simulations, as was done during the first step.

In a last step, the complete synthesis of the filter is carried out, byintegrating the parameters determined at the first and second steps inthe complete structure of the filter. As already mentioned above, it ispossible to choose the distance Pcg separating the first structurePright and the second structure Pleft to be the arithmetic mean of thefirst distance Rg and the second distance Lg. During this last synthesisstep, a new simulation cycle can be carried out in order to verify thatthe filter's response fits the template and optionally to finely adjustcertain parameters.

Of course, the present disclosure is not limited to the embodimentdescribed and variant embodiments can be added thereto without departingfrom the scope of the present disclosure as defined by the claims.Although it was provided here to form the reflection gratings R1, R2 andthe external mirrors 3 a, 3 b using arrays of metal fingers, they mayalternatively be constructed by arrays of grooves etched into thecomposite substrate. These grooves can be etched in the piezoelectricupper layer of the composite substrate and reach the base substrate. Thereflection gratings R1, R2 may also consist of dielectric obstacleshaving a shape identical to the metal fingers considered up to thispoint and deposited during a specific production phase of the filter.The materials of interest to produce these mirrors are silicon dioxide(SiO₂), silicon nitride (Si₃N₄), the combinations of these two materialsmaking it possible to produce so-called oxy-nitrides of specificstoichiometry (of general form SiO_(X)N_(Y)), aluminum oxide (Al₂O₃),aluminum nitride (AlN), zirconium oxide (ZrO₂), hafnium oxide (HfO₂),tantalum pentoxide (Ta₂O₅), and, generally, the oxides may be depositedin the form of a surface layer of the substrates of interest accordingto the microelectronics methods.

1.-20. (canceled)
 21. A surface elastic wave filter with resonantcavities, comprising: a composite substrate formed of a base substrateand a piezoelectric upper layer; at least one input electroacoustictransducer and an output electroacoustic transducer, disposed on theupper layer; at least one internal reflective structure, arrangedbetween the input electroacoustic transducer and the outputelectroacoustic transducer, the internal reflective structurecomprising: a first structure comprising at least one reflection gratinghaving a first period; a second structure comprising at least onereflection grating having a second period, the first period beinggreater than the second period; the filter being characterized in that:the first structure comprises a first plurality of reflection gratingsseparated from each other by a first distance; the second structurecomprises a second plurality of reflection gratings separated from eachother by a second distance; wherein the first distance is less than thesecond distance.
 22. The filter of claim 21, wherein the first structureis distant from the input electroacoustic transducer by a firstseparation distance, the second structure is distant from the outputtransducer by a second separation distance, the first separationdistance being less than the second separation distance.
 23. The filterof claim 22, further comprising two external mirrors arranged on eitherside of the input and output transducers.
 24. The filter of claim 21,wherein the reflection gratings and/or the external mirrors are made byarrays of metal fingers arranged on/in the upper layer of the compositesubstrate.
 25. The filter of claim 21, wherein the reflection gratingsand/or the external mirrors are made by arrays of grooves etched intothe composite substrate.
 26. The filter of claim 21, wherein thereflection gratings and/or the external mirrors are made by arrays ofdielectric fingers arranged on the surface of the composite substrate.27. The filter of claim 21, wherein the composite substrate comprises atleast one layer arranged between the base substrate and thepiezoelectric upper layer.
 28. The filter of claim 21, wherein the basesubstrate has an electrical resistivity greater than 1000 ohm·cm. 29.The filter of claim 21, wherein the thickness of the piezoelectric upperlayer is less than 20 microns.
 30. The filter of claim 21, wherein theinput electroacoustic transducer and the output electroacoustictransducer are respectively composed of two interdigitated combelectrodes.
 31. The filter of claim 21, wherein the distance separatingthe first structure and the second structure is equal to the arithmeticmean, to within 10%, of the first distance and the second distance.